The present invention is related to the treatment of materials in general, and more particularly substrates for microelectronics, optics or optoelectronics. More precisely, the invention relates to a method for transferring a thin layer from a donor substrate to a receiver substrate, the method including the creation of a weakened zone in the donor substrate to delimit a layer to be transferred in the donor substrate. And as will be seen, one particularly advantageous application of the invention is its use in a SMART-CUT® type layer transfer method. According to one particular aspect, the invention also relates to a transfer method for bonding layers under advantageous conditions.
Transfer methods like those mentioned above are already known. These methods are capable of making multilayer wafers using a step of bonding the donor substrate (that includes the layer to be transferred) and the receiver substrate (onto which the layer will be transferred). The subsequent removal of an upper part of the donor substrate finalizes the transfer and forms the final multilayer wafer, that thus includes the transferred layer (for which the thickness is a few hundred Angstroms to a few microns), the layer formed by the receiver substrate (for which the thickness is typically a few hundred microns) and generally an intermediate layer between the two layers mentioned above (and for which the thickness is typically between a few hundred Angstroms and a few thousand Angstroms).
In order to transfer only a thin layer from the donor substrate to the receiver substrate, it is possible to bond the donor substrate to the receiver substrate, and to eliminate part of the donor substrate opposite the layer to be transferred (for example by grinding and/or chemical etching of the exposed face of the donor substrate). In this case, the donor substrate is entirely consumed by the transfer. It is also possible to create a weakened zone within the thickness of the donor substrate, and to bond the donor substrate to the receiver substrate and make the transfer by fracture of the donor substrate at the weakened zone, before bonding the donor substrate with the receiver substrate.
This weakened zone may be obtained by implantation of an atomic species such as hydrogen and/or rare gases such as helium (in the case of the SMART-CUT® technology) or by the formation of a porous zone (in the case of the ELTRAN® technology). Refer to “Silicon-On-Insulator Technology; Materials to VLSI”, 2nd Edition by J. P. Colinge, published by Kluwer Academic Publishers, pages 50 and 51 for a more detailed description of these techniques. This type of transfer technique with the creation of a weakened zone defines the general framework of the invention.
Such substrate manufacturing techniques are typically used to form SeOI type multilayer wafers (semi-conductor on insulator), for which the most common form is SOI (silicon on insulator). When manufacturing such wafers, an insulating layer is formed on the face of the donor substrate and/or the face of the receiver substrate that are brought into intimate contact during the bonding step. This insulating layer may be obtained by treatment of one of the substrates (for example by thermal oxidation) or by deposition. The end result will be the buried insulating layer of the SeOI substrate.
More generally, these transfer techniques can be used to create all types of multilayer wafers, with or without an intermediate insulating layer. The invention is thus applicable for manufacturing all types of multilayer wafers, as will be seen later.
Fracture Zone
It is the that the invention is within the framework of transfer techniques based on fracture at a weakened zone. More precisely, the techniques concerned by the invention are techniques in which the weakened zone is created by introducing one or several species (for example hydrogen or helium) into the donor substrate. Note that in the remainder of this text, the term “species” will be used to denote all cases that arise—including cases in which only a single type of species is introduced. This “introduction” of at least one species may be done by implantation. Other techniques may also be envisaged (for example introduction of species by diffusion—particularly by exposing the donor substrate to a plasma, etc.).
It is always important to define the position of the weakened zone obtained by the introduction of species within the thickness of the donor substrate as precisely as possible. The species introduced into the donor substrate are actually distributed within the thickness of this substrate around a depth corresponding to a concentration peak, with a Gaussian distribution (as a first approximation). The fracture that follows will occur (after inputting energy in thermal or mechanical form, etc.), close to this depth corresponding to the concentration peak of the introduced species—this depth defining a maximum weakening of the substrate.
However, it is observed after the fracture that there is a damaged zone on each side of the fracture plane (and therefore on the upper part of the layer that was transferred onto the receiver substrate and onto the exposed face of the removed part of the donor substrate), and this damaged zone may extend over several tens of Angstroms. These damaged zones may be specifically treated (for example by polishing and/or heat treatment) on the final multilayer wafer or on the remainder of the donor substrate for recycling purposes. And it would be commercially useful to eliminate or at least to minimize such treatments.
Recognizing these problems, U.S. Pat. No. 6,756,286 describes methods designed to minimize the thickness of the damaged zone mentioned above, encouraging the positioning of the implanted species around predefined zones of the donor substrate (these zones may be denoted by the term “inclusions”). This document thus discloses how to:                create an inclusion zone in a substrate by depositing a layer of silicon highly doped with boron on the substrate,        then covering this layer by depositing a layer containing the layer to be transferred,        implanting hydrogen into the donor substrate thus formed, the parameters of the implantation being defined such that a maximum quantity of hydrogen is implanted at the inclusion layer formed by the layer doped with boron. The implanted hydrogen will preferably be located in this layer, due to the chemical affinity.        
This technique significantly reduces dispersion of the hydrogen implanted in the thickness of the donor substrate, and after heat treatment of the fracture it is observed that the damaged zones on the surface of the multilayer wafer created and on the residual donor substrate originating from the fracture are much less significant than with a conventional implantation.
PCT published application WO2004/008514 describes a method for the formation of a weakened zone within a monocrystalline donor substrate by hydrogen diffusion. This method is aimed at forming the weakened zone faster than with existing methods for the formation of a weakened zone by diffusion (see page 5 lines 1 to 4 in WO2005/004514).
US published application US2002/0187619 describes a method for trapping metallic contaminants (see page 1 paragraph [0010] in US2002/0187619). This method includes a step of forming a weakened zone by implantation of species (typically hydrogen) into the donor substrate at a first depth. It also includes the formation of a trapping layer at a second depth.
While these methods are somewhat useful, further improvements are desired, and these are provided by the present invention.
Problems Related to Bonding
Layer transfer techniques include a bonding step during which the surfaces of two layers are brought into intimate contact. In all layer transfer techniques, the bonding quality (in particular characterized by the energy with which the two bonded substrates are bonded to each other) will have a direct impact on the final quality of the multilayer wafer obtained. Thus, the flatness of substrates to be bonded, the presence of particles or contaminants on their surfaces to be bonded, and the extent to which these surfaces are hydrophilic have a direct effect on the energy with which the two substrates will be bonded together after being brought into intimate contact.
It has generally been observed that a sufficiently thick intermediate layer called a bonding layer (typically more than 500 Angstroms thick) placed between two monocrystalline substrates to be bonded facilitates bonding and limits the formation of defects (such as blisters) at the contact interface. Refer to the article “Wafer direct bonding: tailoring adhesion between brittle materials” by Andreas Plossl, Gertrud Krauter, Materials Science and Engineering, #25, Nos. 1-2, Mar. 10, 1999 for further details about this bonding step.
When manufacturing SeOI, the insulating layer that is formed on at least one of the two substrates and that will form the buried insulation of the final multilayer wafer, also facilitates and provides a means of limiting bonding defects. This insulating layer can thus itself form a bonding layer. But there are situations in which it is required to obtain a multilayer wafer that does not include a buried insulating layer. More generally, it may be required to use an intermediate bonding layer (for which it will be noted that the addition will always require an additional step in the method). In this type of situation, it is necessary to transfer the layer directly from the donor substrate onto the receiver substrate, without an intermediate layer (regardless of whether or not it is insulating). This is the case for example when it is required to optimize thermal conduction between the transferred layer (in which the microelectronic components are formed) and the receiver substrate so as to dissipate a maximum amount of heat generated by components during their use.
This is also the case when it is required that the final result should be a wafer in which the transferred layer and the receiver substrate have separate properties without it being required to isolate them electrically. For example, wafers are also known that are formed from a layer of monocrystalline Si associated with a polycrystalline SiC support substrate through an electrically conducting interface. These wafers have been developed to offer an inexpensive alternative to monocrystalline SiC substrates, for different wafer diameters. In making this type of wafer, it is often desirable to bond the Si layer onto the receiver substrate directly.
It may also be desirable to combine a transferred layer of silicon directly onto a silicon substrate, the substrate and the layer having different crystalline orientations in order to optimize performances of transistors that could be formed in the corresponding two elements of the multilayer wafer thus formed. In this respect, refer to the publication “Mobility Anisotropy of electron in inversion layers on oxidized silicon surfaces”. Physical Review, Vol. 4, N6, September 1971 for a description of the differences in the electrical characteristics of silicon depending on its crystalline orientation. It may also be envisaged to make multilayer wafers comprising a thin layer of silicon or Germanium or Silicon Germanium elastically strained in tension or in compression, directly on a solid silicon substrate.
The examples described above may require direct bonding (in other words without an intermediate layer) of two substrates with different crystalline orientations (or more generally different crystalline characteristics). This type of bonding may be problematic since the crystalline structures of the two substrates to be bonded tend to have a mutual influence after bonding, and a region would be created between the two substrates in which the crystalline structure is disturbed. Direct bonding of the two substrates with different crystalline characteristics (for example different crystalline meshes such that the crystalline lattices of the two substrates cannot be aligned) is the cause of crystalline defects that may propagate in one of the substrates, or in the two substrates. In the context of transferring a thin layer from a donor substrate to a receiver substrate to obtain a multilayer wafer, these defects can make the multilayer wafer obtained unusable for the formation of microelectronic components, particularly when they propagate within the thickness of the thin layer.
Note also that there are two main bonding families, for bonding of two substrates. So-called “hydrophilic” bonding requires cleaning operations, before the bonding operation itself, that could facilitate oxidation of the surfaces to be bonded (cleaning with SC1, SC2 type solutions, etc.). So-called “hydrophobic” bonding requires cleaning with an HF type solution. These types of bonding may undoubtedly be used to make direct bonding such as those mentioned above, but these types of bonding are relatively difficult to implement and may also generate defects that will then have to be treated specifically.
When a hydrophilic or hydrophobic bonding technique is used, some species (for example hydrogen molecules (H2) and/or water molecules (H2O) and/or other contaminants) are formed at the contact interface during the fracture heat treatment that takes place at a temperature of the order of 500° C. for a SMART-CUT® type method, while these species disappear at a temperature of the order of 900° C. The presence of these species (H2 and/or H2O and/or other contaminants) at the contact interface induces the formation of “void” type defects or “blisters” at the contact interface between the transferred layer and the receiver substrate. Therefore, a special treatment of these defects is necessary.
Thus, it appears that there is a need for allowing direct bonding of substrates (particularly substrates with different characteristics, for example crystalline). This type of bonding would thus be advantageous when implementing layer transfer techniques, particularly techniques based on fracture at a weakened zone that were mentioned above. Thus, improvements in bonding are also desired.